1. Field of the Invention
The present invention relates to an optical scanning device, which scans an object with a light beam.
2. Discussion of the Related Art
Image forming apparatus using the Carlson process typically perform the following processes:
(1) scanning a rotated photoreceptor drum with a light beam to form an electrostatic latent image thereon;
(2) developing the electrostatic latent image with a developer including a toner to form a toner image on the photoreceptor drum;
(3) transferring the toner image onto a receiving material; and
(4) fixing the toner image on the receiving material, resulting in formation of a hard copy.
Recently, these image forming apparatus have been used for on-demand printing systems (i.e., for simplified printing fields). Therefore, a need exists for an image forming apparatus which can produce high-density images at a high speed.
In attempting to fulfill the need, image forming apparatus, which have a light source capable of emitting plural light beams such as multi-beam laser diodes, and light sources (e.g., vertical cavity surface emitting laser (VCSEL)) in which plural emitting regions are two dimensionally arranged monolithically, have been proposed. The image forming apparatus can scan an object with plural laser beams emitted by the light source, i.e., can perform plural line scanning at the same time.
Optical scanning devices, in which a light source emitting plural light beams is rotated on an optical axis of a scanning optics system of the devices to adjust the beam pitch of the emitted light beams, are used for such image forming apparatus. In this regard, various proposals have been made for such beam pitch adjustment, for example, by published unexamined Japanese patent applications Nos. (hereinafter referred to as JP-As) 2001-13432 and 2002-182141.
When the beam pitch of laser beams is adjusted by rotating the light source unit using the technique disclosed in JP-A 2001-13432, a problem in that when fastening the light source unit using a screw, the unit is unnecessarily moved (rotated), resulting in deviation from the desired position of the light source unit, tends to be caused. Deviation from the desired position of the light source unit causes deviation of position of the light beams (light spots) in the sub-scanning direction, resulting in deterioration of scanning precision of the optical scanning device.
Specifically, in a case where two parts A and B are subjected to positional adjustment, and then the parts are fastened with screws, the following method is generally used. Namely, the method typically includes the following steps: (1) one of the parts (for example, the part A) is fixed; (2) the other part B is born with a chuck, which has an adjustment device and which is connected with the fixed part A, for example, via the ground, and then the position of the part B is adjusted by rotating the part B; (3) after the position is adjusted, the part B is fastened and then the chuck is released from the part B, thereby fastening the parts A and B.
In this case, since the part B, which is a part of a structure, is born with a member (i.e., chuck) other than the parts of the structure, interference is caused due to the force of fastening the part B and the force of the chuck, and thereby variation is caused when the chuck is released, resulting in variation in position of the fastened parts A and B. In addition, when the fastening operation is performed, the two parts A and B are moved relatively due to the torque of the screws, resulting in variation in position of the fastened parts.
Further, in a case where the positions of the two parts A and B are adjusted while the parts are loosely fastened (hereinafter referred to as temporary fastening), the parts can move relatively when starting the fastening operation. In this regard, as the parts A and B are fastened, the parts are minutely transited to the fastened state. Therefore, variation in position (relative position) of the two parts inevitably occurs. In order to reduce such variation, the two parts are preferably fastened as strongly as possible in the temporary fastened state in such an extent that the parts can be relatively moved, and then the positional adjustment operation is performed. However, as the two parts are fastened more strongly, it becomes more difficult to move the parts in the positional adjustment operation.
In this case, if the firmly fastened two parts do not satisfy the desired positional relationship, the parts are loosened again, and then the adjustment operation is performed again. These operations are performed until the two parts satisfy the desired positional relationship. Therefore, the adjustment operation takes a long time, resulting in deterioration of productivity.
In the technique disclosed in 2002-182141, the light source unit is held with a wire material without using a screw clamp mechanism to prevent occurrence of the positional deviation problem of the light source unit. However, in order that the light source unit rotates, it is necessary to form a small gap between the light source unit and the structure. In this case, a problem in that the beam pitch is deviated in an amount corresponding to the gap due to vibration or the like even after the positional adjustment operation tends to occur.
On the other hand, JP-As2000-98278, 2000-75227, 2007-28509 and 2001-228418 have disclosed optical scanning devices in which a laser diode emitting one or more laser beams is held with a holder. The light source unit can be easily rotated via the holder. In addition, the light source unit can be easily subjected to positional adjustment by being moved in the direction perpendicular to the optical axis of the optics system. However, it is difficult to arrange two or more light source units so that the light source units are closely adjacent to each other in the sub-scanning direction. This is because the holder used is relatively large compared to the laser diodes used for the light source unit. Therefore, when the light source units are arranged in such a manner that the laser diodes therein are close to each other, the holders thereof interfere when the laser diodes are rotated. Therefore, in optical scanning devices used for tandem image forming apparatus having plural photoreceptors, the plural light sources have to be separated in the main scanning direction, resulting in jumboization of the image forming apparatus.
Because of these reasons, a need exists for a small sized optical scanning device in which the beam pitch adjustment can be easily performed precisely.